They should just reveal that they are working on the sims5.
EA have done so with several other of their game franchises like Mass Effect, Dead Space and Skate. Those franchises all have games that are being worked on and are not done, some games like Dead Space and Skate are in very early production, but they shared that they are working on them anyway.
https://twitter.com/masseffect/status/1457467946461188100
https://twitter.com/MotiveStudio/status/1436389389958066182
https://twitter.com/skateEA/status/1417167349124304897